In the manufacture and processing of photographic products, diffuse optical transmission density is one of the most important measurements used to characterize the film's properties. The apparatus used to make this measurement are typically referred to as densitometers. A variety of densitometers are known in the prior art. One common feature of many of these densitometers is that the optical density is determined based on calibration and adjustment of the linearity of the detector system. For such densitometers, this calibration must rely on some independent method or reference. Another way to measure diffuse optical transmission density is to determine the optical density with an approximation of the inverse square law. However, a disadvantage of this particular device is that it includes a large mechanical moving system for the light source or the detector which creates an optical instability. Devices are also known to approximate spectral specifications (for example, specifications of ISO 5-3) with one or several appropriate filters. Approximation of such spectral specifications by use of filters introduces error in the measurement result. This error is smaller for samples with spectrally flat transmittance than for samples without spectrally flat transmittance.
U.S. Pat. No. 4,937,764 to Komatsu et al teaches a calibrated densitometer and a method of calibration. A lamp is energized for a predetermined duration of time to illuminate a standard density plate with a spot light formed by a bottom opening of a light tight barrel through an aperture of a transparent plate. The light reflected by the subject sample passes through a measuring aperture and the light tight barrel and reaches a light receiving element through a lens and filter. The light receiving element provides an output corresponding to an intensity of the light received.
U.S. Pat. No. 5,661,556 to Schiff et al teaches a system that measures total integrated scatter from a surface using two integrating devices which can both be integrating spheres or one can be a integrating sphere and the other can be a mirror or lens. This system includes a light source and source optics which direct a beam of light toward the surface. The first integrating device is positioned and configured to receive a first portion of the scattered light which corresponds to a first range of spatial frequencies. The second integrating device is positioned and configured to receive a second portion of the scattered light corresponding to a second range of spatial frequencies. Total integrated scatter data is generated for each range of spatial frequencies and is used to approximate the spectral scatter function of the surface. RMS roughness is then approximated for any range of spatial frequencies.
U.S. Pat. No. 4,900,923 to Gerlinger teaches a reflectance measuring apparatus having a predetermined aperture for the receiving optic. A light-conducting device arranged between the measuring aperture and the specimen enlarges the effective measuring surface of the specimen so that even specimens having a large surface structure can be measured.
U.S. Pat. No. 4,120,582 to DeVries et al teaches an apparatus for testing an optical element sample such as a mirror for determining both the total amount of light reflected from and the total amount of light transmitted by a predetermined area of that optical element sample. The apparatus includes a pair of axially aligned light-integrating spheres between which is clamped the test sample so that no light enters or escapes from either sphere. A substantially collimated beam of light is directed through one sphere against the test sample at an angle to the sphere axis. Silicon photovoltaic light sensitive detectors connected to amplified readout units indicate the total light reflectivity in one sphere and the total light transmission to the other sphere.
Those densitometers of the prior art which do not require independent calibration of the detector linearity are lacking in the ability to determine the diffuse optical transmission density of a sample with a high degree of measuring repeatability and reproducibility. In other words, measuring the same sample on the same apparatus will often result in different measurements for the diffuse optical transmission density of that sample. Further, measuring one sample on two distinct but identical apparatus will also often result in two different diffuse optical transmission densities for that same sample.